Methods of producing aircraft structures comprising the following steps are known:                inserting inside a mould, equipped with a first part and a second part, at least a first sheet of composite material, at least a second sheet of composite material facing the first sheet and spaced apart therefrom, and interconnection elements in a composite sheet material that extend between the first and the second sheet of composite material to delimit an elongated cavity;        inserting a tool (plug) inside the elongated cavity comprising a rigid elongated body delimited by a number of flat faces and having a cross-section corresponding to that of the cavity—the entire length of the rigid body is covered by and enveloped in a tubular sack made of a deformable material;        creating a vacuum in an autoclave inside which the mould is arranged to allow expansion of the sack, which presses against the walls of the cavity during a heating cycle in which the sheets of composite material fuse with each other; and        extracting the tool from the cavity at the end of the heating cycle after having re-established the original pressure—at the end of the first heating cycle, the cavity is delimitated by the rigid walls of a closed-section aircraft structure.        